The use of glycolipid biosurfactants in detergents is well known in the art. Glycolipid biosurfactants include rhamnolipids, sophorolipids, cellobioselipids, trehaloselipids, mannosyl erythritol lipids and (bio)chemical modifications thereof. Glycolipid based biosurfactants are understood to comprise those surfactants that have been obtained through microbial cultivation and consist of carbohydrates bound to aliphatic acids or aliphatic hydroxyl acids through glycosidation or acylation. They offer the advantage of being naturally produced molecules that can be produced through microbial cultivation by feeding renewable raw materials and of being fully degradable after use.
Sophorolipids are one of the most promising glycolipids known, one reason being their high production yield and ease of recovery from the microbial cultivation. Several Candida species, a.o. Candida (Starmerela) bombicola (formerly Torulopsis bombicola), Candida apicola (formerly Torulopsis magnoliae and Torulopsis apicola) and Rhodotorula bogoriensis, Wickerhamiella domericqiae are known to produce sophorolipids in large amounts from various substrates such as carbohydrates, vegetable oils, animal fats and n-alkanes. Sophorolipid production mainly takes place during the stationary phase because of nitrogen limitation. It appears to be enhanced when providing simultaneously hydrophilic (e.g. glucose) and hydrophobic (e.g. fatty acids) substrate to Candida species. Candida bombicola for example produces a complex mixture of 22 structurally different sophorolipids from either glucose and or an oily substrate, preferably C16 to C18 alkanes, fatty acids or their esters. The main compounds produced are the lactonic and the acidic form typically in the ratios of 75 and 25 w/w. A typical hydroxyl fatty acid distribution incorporated in non, mono- and diacetylated sophorolipids either in their free acid or lactonic form is for example disclosed in FR2779057:
TABLE 1TypeHydroxy fatty acidWt. %C16:015-OH hexadecanoic1.5C16:016-OH hexadecanoic2C18:017-OH octadecanoic3.5C18:117-OH octadecenoic60C18:118-OH octadecenoic12C18:217-OH octadecadienoic7C18:218-OH octadecadienoic14
Sophorolipids find numerous applications for example in preventing and curing dandruff and body odor attributed to bacteriostatic properties, as disclosed by EP-A-1.082.097; as therapeutically active substances or cosmetic products, in particular skin treatment as disclosed by EP-A-835.118; and for their antifungal properties disclosed by US-A-2005/0164955. EP-A-820.273 discloses that sophorolipids are non-irritant to the skin, weakly irritant to the eyes only and that they show anti-inflammatory as well as elastase inhibiting activity. Other uses of sophorolipids include their use as the sole surfactant in washing and cleaning applications such as disclosed in U.S. Pat. No. 6,433,152. According to EP-A-1.445.302 mixtures of sophorolipids show synergistic activity in laundry and hard surface cleaning applications. EP-A-499.434 discloses to combine sophorolipids with lamellar, usually ethoxylated non-ionic surfactants in washing and cleaning applications. DE-A-19600743 discloses manual dishwashing formulations based on sophorolipids and other glycolipids combined with high foaming surfactants.
However, the physicochemical properties of sophorolipids form an important limitation to their applicability. In this respect it is mentioned that the lactonic form of the sophorolipids is hardly water soluble, it is insoluble in acidic environment, it is spontaneously deacetylated in alkaline pH and upon deacetylation the pH reduces to approximately 6. Fully de-acetylated acidic sophorolipids appear to lose a substantial part of their surface activity with respect to the di-acetylated lactonic form of crude sophorolipids. Shorter-chained sophorolipids are presumed to decrease surface tension more effectively than the known long chain C16-C22 sophorolipids. Therefore several attempts have been made to find an efficient process for their production. Whereas, Candida bombicola and its most commonly used strain (ATCC 22214) have been found capable of readily fermenting C16 and C18 chains and of incorporating them into the glycolipids, shorter hydrocarbon chain precursors C10-C14 were found to be hardly incorporated by the micro-organism: they were assimilated instead. C15-sophorolipids could only be obtained in unsatisfactory yields of below 40% by Jones and Howe (1968), by offering a wide variety of expensive C15 substrates to Torulopsis gropengiesseri. Pentadecanoic acid proved to be too short for appreciable hydroxylation, which is an essential step in sophorolipid production by the micro-organism, and 12-hydroxyoctadecanoic acid did not react because the hydroxyl group is too close to the reaction site.
Traces of a C14 component could only be detected when a glucose/tetradecane mixture was used as hydrocarbon source. According to the teaching of U.S. Pat. No. 6,433,152 short chain 3-alkanols, 4-alkanols and 2, 3 and 4-alkanols could be converted into short chain sophorolipids with the envisaged chain length, provided the conversion was carried out under reduced oxygen concentrations and with an exceptionally high glucose feed. However, the yield was only between 3-17 g/l, of which only 41-85% had the envisaged short chain length. Although U.S. Pat. No. 6,433,152 claims incorporation of the easily obtainable and cost-effective 1-alcohols, no examples are given or yields specified. It therefore seems likely that 1-alcohols do not form a useful source for the production of short-chain sophorolipids, as these are first converted into the corresponding acids before they are transformed into glycolipids rather than being glycosidically bound at the hydroxyl group (Jones et al, 1968). According to the disclosure of DE19518982.5 Candida species are capable of converting secondary alcohols or ketones to non-cyclic sophorose lipids with surface active properties. The yields amount to approximately 20 g/l of C12-C14 sophorolipids, 75-85% of which has the desired chain length. 7% was converted into a fatty diol. However, the low yield, combined with the impurity of the end product which is a mixture containing 10% of standard C18 sophorolipids and the fact that secondary alcohols are expensive, render this process unattractive.
In most Candida species, substrates having a hydrocarbon chain of more than 18 carbon atoms, are first reduced in chain length by one or more units of two carbon atoms and only thereafter hydroxylated, thus yielding C16 or C18 hydroxy fatty acids with the OH group on the penultimate carbon atom (Tulloch, Spencer and Gorin 1962).
Van Bogaert et al (2007) describe the cloning, characterisation and functionality of the orotidine-5′-phosphate decarboxylase gene (URA3) in Candida bombicola. 